This disclosure relates to touch sensitive controls in general, and in particular, to communication headsets having improved electrostatic discharge (ESD) prevention and donned/doffed detection capabilities.
Touch sense controls function by measuring a change in a parasitic capacitance caused by a change in the proximity of a human body to a sensor electrode. When the body moves very close to the electrode, the change in the series combination of the body-to-sensor ground capacitance and the body-to-electrode capacitance is coupled to the sensor, which then acts on the change in capacitance to: 1) detect the proximity or remoteness of the body relative to the electrode; and, 2) effect some control function in response to the proximity or remoteness of the body detected, e.g., activating/deactivating an electrical circuit. The total body-to-electrode capacitance depends on the area of the body in proximity to the electrode. Generally, the smaller the area of the body in proximity to the sensor, the less is the capacitance.
FIG. 1 is a schematic illustration of a communications headset 10 equipped with a conventional “DON/DOFF” sensor 12 that detects whether the headset has been “donned,” i.e., placed on the head of a wearer for communication purposes, or “doffed,” i.e., removed from the wearer's head. The headset includes a metal or metalized earpiece 14 that is placed within or directly against the ear 16 of the wearer for listening purposes, and which also functions as the electrode of the DON/DOFF sensor 12. The body-to-sensor ground capacitance is represented by C1 and the body-to-electrode capacitance at the point of contact 18 between the electrode 14 and the wearer's ear 16 is represented by C2. Thus, the series combination of the parasitic capacitances coupled to the sensor 12 is given by C1+C2, which the sensor acts upon to determine whether the headset 10 has been donned or doffed by the wearer.
Generally speaking, the sensitivity of a touch sensor 12 increases when the change in the series parasitic capacitance caused by a touch is maximized. One way to maximize the change in parasitic capacitance is by making the area of contact between the body and the sensor electrode 14 as large as possible. Another way is by bringing the body (for example a finger or an ear) as close to the sensor electrode as possible.
The body-to-sensor capacitance C1 is usually relatively large, so that it is the body-to-electrode capacitance C2 that changes the greatest amount with a touch of the sensor electrode 14. In fact, if the body contacts the metal electrode directly, the maximum change in parasitic capacitance will occur. However, direct contact of the body with the electrode can lead to a number of problems, in that, if the metal electrode comes in direct contact with the wearer's skin, a sudden electrostatic discharge (ESD) may occur, which can cause an uncomfortable shock to the wearer. Furthermore, long term contact between the metal of the electrode and the wearer's skin can result in both skin irritation and corrosion of the electrode, due to the presence of moisture and oils in the skin.
Accordingly, it is conventional to isolate the touch sense electrode 14 from direct contact with the wearer's skin with an electrical insulator, such as a soft plastic or foam rubber covering 20, such as that illustrated in FIG. 1. While this covering 20 overcomes the ESD, skin irritation and electrode corrosion problems outlined above, it also results in an increase in the displacement between the wearer's skin and the electrode 14, thereby reducing the sensitivity of the sensor 12.
As an additional consideration, in the case of a DON/DOFF sensor applied to an earpiece that is held inside of the ear cavity, e.g., a so-called “in-canal” earpiece 14, the housing of the ear-piece that is introduced into the ear typically comprises or is plated with a metal, and is then covered with the electrically insulating cover 20. Ear skin contact is therefore neither complete nor consistent from wearer to wearer, which necessitates covering of the entire earpiece with the cover. However, even if the entire earpiece is covered, the actual contact area between the earpiece 14 and the wearer's ear 16 is often relatively small, thereby reducing sensor sensitivity.
In addition to the reduction in sensor sensitivity caused by the ear-electrode separation resulting from the plastic cover 20 and the inconsistent electrode contact problem, some earpieces require an air gap (not illustrated) between the cover 20 and the earpiece housing 14 for reasons of acoustic efficiency, thereby further reducing sensor sensitivity.
Accordingly, there is a long-felt but as yet unsatisfied need for touch sensitive controls that avoid the above ESD, skin irritation and electrode corrosion problems, yet which also have improved sensor sensitivities relative to those of the prior art.